Exclude affecting of DB schema changes on the Fuel CI jobs for fuel-stats

When we have changed DB schema other patchsets failed due to Alembic can't upgrade/downgrade DB.
We need to have fallback mechanism in the DbTest setUp for cleanup all indexes/sequences/tables by hands if Alembic doesn't able to downrade DB schema.

Steps for reproduce:

- Clone fuel-stats:
git clone https://github.com/openstack/fuel-stats.git

- Configure development env: https://review.openstack.org/#/c/294673/2/README.rst (How to configure development environment)

- Create branch:
git branch test_branch

- Apply latest DB migration:
./manage_collector.py --mode test db upgrade -d collector/api/db/migrations/

- Create DB migration:
./manage_collector.py --mode test db migrate -d collector/api/db/migrations/

- Apply migration:
./manage_collector.py --mode test db upgrade -d collector/api/db/migrations/

- Remove created migration file from the collector/api/db/migrations/

- Switch back to master:
git checkout master

- Try to run test:
tox -epy27

We had an error: CommandError: No such revision '3c3a4fa3a39f'